In many situations it can be desirable to image the interior of opaque objects. By way of example but not limitation, in the medical field, it can be desirable to image the interior of a patient's body so as to allow viewing of internal structures without physically penetrating the skin.
Computerized Tomography (CT) has emerged as a key imaging modality in the medical field. CT imaging systems generally operate by directing X-rays into the body from a variety of positions, detecting the X-rays passing through the body, and then processing the detected X-rays so as to build a three-dimensional (3D) data set and a 3D computer model of the patient's anatomy. The 3D data set and 3D computer model can then be visualized so as to provide images (e.g., slice images, 3D computer images, etc.) of the patient's anatomy.
By way of example but not limitation, and looking now at FIGS. 1 and 2, there is shown an exemplary CT imaging system 5. CT imaging system 5 generally comprises a torus 10 which is supported by a base 15. A center opening 20 is formed in torus 10. Center opening 20 receives the patient anatomy which is to be scanned.
Looking next at FIG. 3, torus 10 generally comprises a fixed gantry 22, a rotating disc 23, an X-ray tube assembly 25 and an X-ray detector assembly 30. More particularly, fixed gantry 22 is disposed concentrically about center opening 20. Rotating disc 23 is rotatably mounted to fixed gantry 22. X-ray tube assembly 25 and X-ray detector assembly 30 are mounted to rotating disc 23 in diametrically-opposing relation, such that an X-ray beam 40 (generated by X-ray tube assembly 25 and detected by X-ray detector assembly 30) is passed through the patient anatomy disposed in center opening 20. Inasmuch as X-ray tube assembly 25 and X-ray detector assembly 30 are mounted on rotating disc 23 so that they are rotated concentrically about center opening 20, X-ray beam 40 will be passed through the patient's anatomy along a full range of radial positions, so as to enable CT imaging system 5 to create a “slice” image of the anatomy penetrated by the X-ray beam. Furthermore, by moving the patient and CT imaging system 5 relative to one another during scanning, a series of slice images can be acquired, and thereafter appropriately processed, so as to create a 3D data set of the scanned anatomy and a 3D computer model of the scanned anatomy. In practice, it is common to configure X-ray detector assembly 30 so that multiple slices of images (e.g., 8 slices, 16 slices, 32 slices, etc.) may be acquired with each rotation of rotating disc 23, whereby to speed up the acquisition of scan data.
In practice, it is now common to effect helical scanning of the patient's anatomy so as to generate a 3D data set of the scanned anatomy, which can then be processed to build a 3D computer model of the scanned anatomy. The 3D data set and 3D computer model can then be visualized so as to provide images (e.g., slice images, 3D computer images, etc.) of the patient's anatomy.
The various electronic hardware and software for controlling the operation of rotating disc 23, X-ray tube assembly 25 and X-ray detector assembly 30, as well as for processing the acquired scan data so as to generate the desired slice images, 3D data set and 3D computer model, may be of the sort well known in the art and may be located in torus 10 and/or base 15.
In many cases CT imaging system 5 is intended to be stationary, in which case base 15 of CT imaging system 5 is set in a fixed position on the floor of a room and a special motorized movable bed is provided to move the patient relative to CT imaging system 5 during scanning. More particularly, with a stationary CT imaging system 5, the patient is brought to the location of CT imaging system 5, the patient is placed on the motorized movable bed, and then the motorized movable bed is used to move the patient relative to CT imaging system 5 (i.e., to advance the patient into center opening 20 of CT imaging system 5) so that some or all of the length of the patient may be scanned by CT imaging system 5.
In other cases CT imaging system 5 is intended to be mobile so that the CT imaging system may be brought to the patient and the patient scanned at the patient's current location, rather than requiring that the patient be transported to the location of the CT imaging system. Scanning the patient with a mobile CT imaging system 5 can be highly advantageous, since it can reduce delays in patient scanning (e.g., the patient can be scanned in an emergency room rather than waiting to be transported to the radiology department) and/or it can allow the patient to be scanned without requiring movement of the patient (e.g., the patient can be scanned at their bedside in an intensive care unit, or “ICU”). To this end, and looking now at FIGS. 4 and 5, base 15 may comprise a transport assembly 50 for (i) moving mobile CT imaging system 5 to the patient prior to scanning and (ii) moving the CT imaging system relative to the patient during scanning. More particularly, transport assembly 50 preferably comprises (i) a gross movement mechanism 55 for moving CT imaging system 5 relatively quickly across room distances, so that the CT imaging system can be quickly and easily brought to the bedside of the patient, such that the patient can be scanned at their bedside without needing to be moved to the radiology department, and (ii) a fine movement mechanism 60 for moving the CT imaging system precisely, relative to the patient, during scanning so that the patient can be scanned on their bed or gurney without needing to be moved onto a special motorized movable bed. In one preferred form of the invention, gross movement mechanism 55 preferably comprises a plurality of free-rolling casters 62, and fine movement mechanism 60 preferably comprises a plurality of centipede belt drives 63 (which can be configured for either stepped or continuous motion, whereby to provide either stepped or continuous scanning of the patient). Hydraulic apparatus 65 permits either gross movement mechanism 55 or fine movement mechanism 60 to be engaged with the floor, whereby to facilitate appropriate movement of mobile CT imaging system 5. Thus, with a mobile CT imaging system 5, the CT mobile imaging system may be pre-positioned in an “out of the way” location (e.g., in an unused corner of an emergency room) and then, when a patient requires scanning, the patient may be quickly and easily scanned at their bedside, by simply moving the mobile CT imaging system to the patient's bedside on gross movement mechanism 55 (e.g., casters 62), and thereafter moving the mobile CT imaging system during scanning on fine movement mechanism 60 (e.g., centipede belt drives 63).
In current CT imaging systems, X-ray detector assembly 30 typically comprises solid state or ceramic detectors.
A CT scanner equipped with solid state or ceramic detectors needs to be calibrated for two separate effects. The first is the calibration of the “beam hardening” effect due to the polychromatic nature of the X-ray spectrum. The second calibration is to compensate for variations in the spectral response of the detector caused by impurities in the detectors. Typically, the calibration process involves scanning well known materials (such as plastic or water) and determining an adjustment factor for each detector which reflects proper calibration for that detector. Some existing methods use cylindrical water phantoms of several diameters coupled with very complex mathematical procedures. Other calibration methods use solid slabs of well known materials.
The advantage of the water calibration method is its ability to deal with a low grade material. However, a successful water-based calibration requires several water phantoms and a special calibration “seed” that needs to be generated manually and fine-tuned for each scanner. As such, water calibration is a lengthy procedure.
Slab-based calibration is faster and requires a less complex mathematical algorithm. But the success of the slab-based calibration depends on the quality of the detector array, i.e., the similarity or the closeness of the spectral responses of the various detectors. This typically requires a costly process of “culling” detectors which significantly vary from a “norm”, which increases the cost of the detectors. Slab-based calibration also requires specially designed slabs that minimize the beam hardening effect through the material of the slab.
Thus there is a need for an improved detector calibration process.